The present invention relates to Microelectromechanical Systems (MEMS). More particularly, the present invention pertains to frequency selective MEMS devices, and methods for manufacturing MEMS devices.
Currently, there is an interest in increasing the degree of integration of electronics. Integration has proceeded steadily over the last few decades and achieved remarkable reduction in the physical size occupied by electronic circuits. Semiconductor lithography, has enabled circuits with millions of transistors to be constructed on a single silicon die. Nonetheless, certain components are difficult to integrate.
For example, inductors are difficult to integrate Although certain spiral shaped designs for integrated circuits have been proposed, owing to their inherent resistive losses, these spiral inductors are ill suited for producing high Q resonators which are needed to generate stable frequency signal sources.
One important component that is used to generate stable frequencies in a variety of electronic apparatus including sequential logic (e.g., microprocessors) and wireless communication transceivers is the quartz crystal resonator. The quartz crystal resonator in its usual form is a bulky discrete component.
Microelectromechanical System (MEMS) based resonators have been proposed as an alternatives to quartz resonators for use as frequency selective components for use at RF frequencies. One type of MEMS resonator that has been proposed comprises a suspended beam of semiconductor material that is shaped and sized to resonate at a selected frequency chosen in view of a desired electrical frequency response. The MEMS resonator serves as a frequency selective component in a circuit. According to one design the MEMS resonator is driven by a drive electrode that extends below the suspended beam. Electric force interaction between the suspended beam and the drive electrode induces the suspended beam to vibrate.
Although a MEMS resonator occupies very little space compared to an external discrete component it does take up substantial space compared to electrical components found on integrated circuits. A single MEMS resonator can take up space on a semiconductor die that could have been used for tens of transistors. In some applications it would be advantageous to be able to reduce the die area occupied by a MEMS resonator.
Another drawback of suspended beam type MEMS resonators is that they are susceptible to shock and vibration. External shock and vibration will cause spurious electrical signals to be generated by beam type MEMS resonators.
During the past decade there has been an increased interest in the semiconductor industry in use of Silicon On Insulator (SOI) wafers. SOI wafers include a silicon substrate, a silicon di-oxide layer on the silicon substrate, and a single crystal silicon layer on the silicon di-oxide layer. SOI wafers afford a number of advantages in terms of the electrical properties of circuits built using them, including reduced voltage requirements, and power consumption for a given clock speed.
It would be advantageous to have a MEMS resonator design that is especially suited for implementation on a SOI wafer.
The electrical impedance of a beam type MEMS resonator is determined by its geometry. It would also be advantageous for some applications, to be able to provide a MEMS resonator having reduced impedance.